284. =Winds as Affected by the Rotation of the Earth.=--The heat of the vertical sun upon the tropics causes a rise of heated air into the upper regions, while there is a rush of colder air toward the equator from both north and south. This effect is represented in Fig. 200 (p. 221), E being the sun, N the north pole, and S the south pole. An effect similar to that represented in Figs. 198 and 199 is produced here, but it is on a much larger scale. But the diagram does not present the matter in its true light in all respects. The prevailing winds in the equatorial regions are not north and south winds, as would appear from this diagram; but they are from the northeast and southeast. I will explain this by Fig. 201. As the earth turns on its axis it is plain that there is no part of the surface of the earth that moves so rapidly as the equator, E W, for that moves in a larger circle than any other part. And the nearer you go to either pole, N or S, the less is the rapidity of the revolution. Now the atmosphere, as stated in § 188, partakes of the motion of the earth. The air, therefore, at the equator is moving from west to east with the rotation of the earth faster than it is any where else, and the nearer you go to either pole the slower is its motion. It follows from this that any portion of air blowing from the north or the south toward the equator, as it comes from where it was moving east slower than air at the equator is, would from its lesser momentum lag behind the air of the equator, the wind would be curved toward the westward, as indicated by the arrows. The result would be that the northern wind would be converted into a northeaster, and the southern into a southeaster. All this can be made more clear with a globe, or, indeed, with any round object. 285. =Liquefaction.=--The change of solids into liquids is one of the most observable effects of heat. This change requires different degrees of heat in different substances. Thus while iron melts at the high heat of 2786°, lead melts at 633°, sulphur at 239°, ice at 32°, and mercury at 39° below zero. Mercury is never found in a solid state, but it sometimes becomes solid in the arctic regions when carried there and exposed in the open air. We are apt to think of water as being in a more natural state when liquid than when it is solid, just as we think of iron as being naturally solid and mercury as naturally liquid. But in all these cases the state of the substance depends on its temperature, and this is varied by circumstances. Water at the equator is always liquid, and the idea of ice there is exceedingly unnatural; while near the poles it is the reverse, ice and snow reigning every where throughout the whole year.